1. Field of the Invention
This invention relates generally to gas turbine control systems and more particularly to a control system and method of controlling fuel flow to a gas turbine to control the temperature thereof.
2. Description of the Prior Art
The present day high acceptance of large capacity gas turbine power plants for large peak load applications has made the reliability and availability of these power plants an important concern to the user. This is due in large part to the fact that these power plants are often located in remote areas and operate unattended. Remote control is sometimes provided by a telemetry communication link.
To assure higher turbine reliability, turbine temperature regulation must be accurate. More importantly, however, is that the system regulating the temperature must be more available to the turbine. That is, the system should continue to operate the turbine even if some turbine control function or condition of a noncatastrophic nature fails. A system which continues to run, even with some failures, is said to have fail-soft design, whereas a system which is shutdown with any failure is said to have fail-safe design.
Traditionally, the operating temperature of a gas turbine has been controlled by regulating either the firing or exhaust temperature of the turbine. Regardless of which is to be regulated, multiple thermocouples or other temperature sensing devices are employed for sensing the turbine temperature. For example, to measure the exhaust temperature, a plurality of thermocouples are placed at various points in the turbine exhaust plenum and their conductor leads are connected together in order to obtain an average exhaust temperature.
In the prior art, this averaging has been done by electronic analog elements interconnected as a temperature regulating system to control the average exhaust temperature at some predetermined level.
Analog temperature regulators present several problems:
1. A temperature reference point, usually an analog reference voltage, is employed in analog controllers for comparison with the average exhaust temperature to control the turbine temperature at a predetermined level. It is well known in the art that these analog reference voltages are susceptible to drift with time and/or temperature, thus leading to inaccuracies in temperature control. PA1 2. Wiring the thermocouples wires together to obtain an averaged temperature signal is an undesirable situation. For example, if a thermocouple fails with a lower than specified output (i.e., shorted wiring, falsely indicating a cold spot in the exhaust plenum) the averaged exhaust temperature will falsely indicate a lower exhaust temperature. As a result, the regulating action of the analog control will cause the exhaust temperature to increase to a value higher than the predetermined level. Such an increase in temperature could seriously degrade the life of the turbine. PA1 3. By hardwiring the thermocouples together, it is difficult to electronically evaluate the operational validity or reliability of each thermocouple. As such, no information is available to detect for potentially abnormal turbine combustion problems, failed thermocouples, or bad wiring. PA1 4. Finally, it is difficult to design a fail-soft turbine temperature regulator or control system without incurring excessive costs by the essential implementation of redundant analog temperature regulators and components for the isolation of potential combustion problems.
It is, therefore, desirable to provide a control system and method for controlling the operating temperature of a gas turbine which enhances turbine life by either shutting the turbine off in the event of a catastrophic failure or allowing the turbine to continue running even though failures of a noncatastrophic nature occur in the system.